Calcined kaolin pigments have found increasing acceptance in paper manufacture, where they are particularly useful as fillers in or for coating of paper and paperboard products. Such pigments are also widely used as fillers in paints, rubbers and plastics. In the usual process for preparing such a pigment, a selected kaolin crude, after mining, is generally worked up by washing, classifying, and if desired, by other types of beneficiation. After such treatment the kaolin results as an aqueous slurry. The slurry is then dried, typically by spray drying, may then be pulverized, and is calcined. The present invention relates particularly to the spray drying/calcining steps and to the integration of these steps.
The general techniques thus utilized in preparing calcined kaolins are discussed in a number of prior art patents. For example, Fanselow et al, U.S. Pat. No. 3,586,523 describes the production of a calcined kaolin filler pigment, in which the starting material, a fine particle-sized kaolin crude is crushed, dispersed in water, degritted, classified to remove nearly all particles larger than about 2 microns ESD (equivalent spherical diameter), and chemically bleached with or without flotation beneficiation. Specifically, the filter cake from bleaching containing about 60% solids, is fluidized by adding a small amount of ammonium hydroxide. The dispersed slip is spray dried, pulverized, calcined and again pulverized. This patent therefore describes some basic operations in the production of calcined kaolin pigments, but nothing is said about using the calciner hot off-gases for any purpose.
This is also true of McConnell et al, U.S. Pat. No. 4,381,948, which describes the preparation of a kaolin clay pigment consisting of high light scattering porous aggregates of kaolin clay platelets, having certain properties of specific gravity, porosity and size distribution. The method of preparation typically comprises forming an aqueous dispersion of the crude clay at e.g. 60% solids, diluting with water to 15% solids, degritting, and subjecting to a particle size separation by centrifuging. From such separation a kaolin slurry of clay is recovered in which substantially all particles are less than 1 micron ESD. The slurry is dried by spray drying, pulverized, calcined under controlled temperature conditions, and again pulverized.
In Podschus et al, U.S. Pat. No. 3,309,214, a process is disclosed for the preparation of calcined kaolin pigments from kaolin clays. The crude kaolin is worked up by washing, classifying, etc., and after such treatment results as an aqueous slurry of about 50% by weight solids. It is then subjected to two calcining treatments, the first of which is known as "shock calcination", and the second of which is of the conventional type. Shock calcination is effected in a highly turbulent hot gas stream introduced into a conical reaction chamber at high velocity, in which the kaolin is heated to a temperature between 600.degree. to 1000.degree. C. for a period of not more than one second. The second calcination is effected at a temperature between about 800.degree. to 1100.degree. C. for a time of at least 10 minutes, e.g. one hour, in a conventional furnace such as a muffle furnace. Prior to said first calcination, the kaolin may be dried, preferably by the hot gases of the shock calcination furnace, at temperatures below about 400.degree. C. to about 500.degree. C. as it is desirable to prevent the kaolin from being dehydrated, since the shock calcination of kaolin which is already partially dehydrated is less effective. That is, the heating of the wet kaolin may be effected in two stages instead of in a single stage, but in both instances by the same source of heat. It may be noted that there is no cooperation between the second calciner and a spray drier (none is used), and no fuel savings are realized from the off-gas of the second calciner.
An article by Paul A. Hubert in Pulp and Paper, February, 1980, describes the use of a flash drier with a lime kiln. In the flash-drier arrangement, washed lime mud containing 30% to 40% moisture is conveyed from the recovery system centrifuge or vacuum filter (normally the final stage before drying and calcining in the kiln) to a mixer, where it is combined with a portion of dried material from the system cyclone. By properly blending dry and wet materials, the feed is conditioned for more efficient drying while eliminating the handling problems associated with wet calcium carbonate sludge. The conditioned feed is then discharged into the flash drier-cage mill where it contacts the hot gas stream from the cold end of the kiln. These gases are now at 1000.degree. F. to 1200.degree. F., hotter than normal. Because dry material is being fed to the kiln, the drying operation has been moved from the kiln to the flash drier. The cage mill disintegrates lumps of calcium carbonate, mixing the hot gas stream with the fine particles. The hot gas stream conveys the material through the uptake duct to the cyclone, simultaneously drying it to 1% moisture. The dry material is separated from the air stream in the cyclone and discharged into a dry divider where a portion is returned to the mixer for conditioning the feed. The remainder is fed to the kiln for calcination. Since it is necessary to split the dried material and return one portion to the mixer where it contacts wet feed so as to condition the wet feed for more efficient drying, it follows that there is recycling of some dried material through the flash drier. Fuel savings are said to be just under 10% to over 30%. On the other hand, in the subject process, the process steps are not comparable and fuel savings can be in a higher range, viz., 40-50%. Furthermore, the calcium carbonate feed is not a kaolin, and does not have the properties of a kaolin.
Mixon, Jr., U.S. Pat. No. 4,246,039 discloses the wet processing of kaolin in which an electrically augmented vacuum filter, designated "EAVF", is used to partially dewater an aqueous slurry. In the process, a kaolin slurry containing less than 45% solids is preheated by direct heat exchange with hot stack exhaust from a clay calciner in a wet scrubber. The exhaust gas is typically in the range of 800.degree. to 1000.degree. F. and the slurry is heated from ambient temperature to a temperature above 100.degree. F., e.g. in the range of 125.degree. to 140.degree. F., with some water being evaporated to increase solids by 2 to 5 weight percent. The use of preheated feed to the electrofilter is said to reduce the energy demands of the electrofilter and as preconcentrated slurry is discharged from that apparatus and fed to the spray drier at elevated temperature, to reduce energy requirements to operate the spray drier. The exhaust gas from the kiln contains small amounts of particulates, including fine calcined clay particles which are introduced into the clay slurry in the wet scrubber. However, the EAVF filter is a complex means of removing water. Its operation involves electrolytic dissociation of the water. Hydrogen gas, sodium ions and hydroxide ions form at the cathodes. Oxygen and hydrogen ions accumulate at the anodes. Clay collects on the anode surfaces. The apparatus includes an overhead traveling carriage equipped with hoists to raise an anode vertically out of the slurry and return it vertically to the slurry. Doctor blades associated with the traveling carriage are constructed to scrape the clay filter cake from the surfaces of the anodes before they are resubmerged in the clay suspension. Over long-term use it would be subject to breakdown/repair.
Conventionally, however, the exhaust gas from the calciner is fed to a wet scrubber which takes out the dust particles and the gas is vented. Wet scrubbing is resorted to in order to meet governmental dust attainment requirements for the vented air. However, the dust so removed is in the form of a very dilute aqueous suspension, and although the suspension could theoretically be filtered and the product dried, such process is uneconomical. Accordingly, the suspension is discharged to waste, thereby adding to the cost of waste disposal.
In principle, it might be thought that one could directly collect the dust in the calciner exhaust gas by means of a conventional bag dust collecting system, but the fabrics used in such collectors cannot withstand the high temperatures which would be encountered at the exhaust from the calciner, typically about 1000.degree. F.
Accordingly, the present invention is directed to a practical method and system, which functions to reduce the energy requirements associated with the wet processing of kaolin to dry, calcined particles, and which further, provides for recovering the fines in the calciner exhaust gas.